One concept of an electric energy store provides for using a metal in conjunction with an air electrode. What may be used as an oxygen-conducting solid electrolyte is, for example, yttrium- or scandium-stabilized zirconium dioxide. These electrolytes exhibit highly selective oxygen conduction, but require relatively high operating temperatures of typically more than 600° C.
Since air is required at least temporarily for operating an air battery and also has to be brought to a minimum temperature, the aim is to keep the mass flow of air as low as possible and offer only as much air as is actually needed for the electrochemical reaction. Heat losses which would inevitably occur because of the discharge of the hot exhaust gas into the environment can thereby be minimized. Only some of the heat from the exhaust gas stream can ever be recovered efficiently.
The constantly occurring temperature gradients which lead to thermal stresses present a technical challenge in high temperature air batteries. Depending on their type of construction, therefore, the temperature gradients should not overshoot a critical value, so as not to damage the stack (as a rule, a stack of several individual cells). In addition to thermal stress, however, the absolute temperature may also have a limiting effect. Calculations have shown that the temperature of the gas stream may rise above a value of 1000° C., theoretically even above 1500° C., if a correspondingly low air stream is selected. Such high temperatures are undesirable because there are no cost-effective materials for such temperatures and also major heat losses through the insulation would occur.
A further technical challenge arises from the fact that the batteries often have to dwell in standby mode. The batteries often in this case have to fulfill the requirement that they must become operative within a few minutes and in this case give virtually maximum power in the shortest possible time. For this reason, the battery should not cool in standby mode and should be maintained constantly at the approximate operating temperature.